Internal pressure molding (IPM) is often adapted for forming a shaft such as a golf club shaft. In the molding process, a mandrel is covered with an internal-pressure holding tube (bladder), a prepreg made of a sheet FRP is put on the mandrel covered with the internal-pressure holding tube, the composite body that is composed of the mandrel, the bladder and the prepreg is set in a shaft forming die, and the shaft forming die is heated while the prepreg and the shaft forming die are brought into intimate contact with each other by applying pressure to the inside of the internal-pressure holding tube to increase the internal pressure thereof. The mandrel is either used as an air passage during the heating of the shaft forming die, or removed after the aforementioned composite body is completed so that an air-injecting mouth piece, which is provided independently of the mandrel, can be fixed to an open end of the internal-pressure holding tube.
However, in the above-described internal pressure molding process, the air exiting a space between the shaft forming die and the prepreg prevents the shaft forming die and the prepreg from being in intimate contact with each other. This makes it impossible to obtain a satisfactory surface irregularity (surface roughness) of the prepreg. To prevent this problem from occurring, the assignee of the present invention has applied a vacuum forming method (which is a known forming method in which the air in a forming die is evacuated to a degree of vacuum), which is often used as a vulcanization forming for rubber material, in an internal pressure molding process. Nevertheless, it has still proved difficult to achieve a satisfactory surface irregularity.